Generally, a frequency converter for satellite communication wherein reception signals of the SHF band are fed through a waveguide is composed as shown in FIG. 1. That is, it consists of input flange unit 1 through which the SHF signal from the broadcast satellite is first applied, rectangular waveguide unit 2 which is provided in the final end of the rectangular waveguide and possesses a rectangular waveguide-microstrip line converter unit 3, microwave integrated circuit (MIC) 4 which is housed in a shield case 5 and which is responsible for processing of reception signals, and other attached circuits 6. Reference, numeral 7 designates an output terminal, and numeral 8 is an outer case which comprises the rectangular waveguide unit 2 having said rectangular waveguide-microstrip converter unit 3, microwave integrated circuit 4, shield case 5 and attached circuits 6.
In this construction, the SHF signal from the satellite supplied from the input flange unit 1 propagates through the rectangular waveguide unit 2 and enters the microwave integrated circuit 4 by way of rectangular waveguide-microstrip line converter unit 3. In this microwave integrated circuit 4, the signal is amplified and frequency-converted, and is delivered from the output terminal 7 as a signal of intermediate frequency.
As well known, the frequency converter for satellite communication is required to have a large image suppression ratio, small leakage of local oscillation frequency, and low noise index. In this respect, in the conventional constitution as shown in FIG. 1, the image suppression ratio and leakage of local oscillation signal were forced to depend only on the microwave integrated circuit 4, and there was a limit to the circuit design and satisfactory performance could not be obtained.
Another example of a conventional frequency converter is shown in FIG. 2, wherein the same numbers designate to the same parts as in FIG. 1. The converter in FIG. 2 is furnished with a waveguide type filter 9 in the rectangular waveguide unit 2 in order to satisfy the requirements of image suppression ratio and local oscillation signal leakage which were the defects in the conventional example shown in FIG. 1. The other construction is similar to that of FIG. 1.
In this construction, the performances of the image suppression ratio and local oscillation frequency may be sufficiently satisfied, but the insertion loss of said filter 9 exerts adverse effects on the noise index of the frequency converter, which is a serious defect.